Bearing in mind that many people regard a TP as a pilot who speaks fluent Engineer (and conversely, an FTE as an Engineer who speaks fluent pilot), I believe from occasional conversations with people at RR that the main function of their test pilots is to be
(a) Competent Test Pilots, and
(b) Fully conversant with all of RR's products and specific engine testing requirements.
So that the company can provide direct, competent, input into aircraft manufacturer's test programmes, ensuring that RR gets the testing it requires done, and the data it needs back.
Similarly, if an engine problem is found in-service, surely the most effective way that RR can gather the information it requires to solve that is to send it's own competent TP(s) to fly with the operator (or at the very least, discuss in depth what's going on).
Please take this post in the light that I've never worked for RR, and on the odd occasions I've tested RR products it was as an Engineer and we used our own TPs in consultation with RR engineers - but that's my understanding of why they have their own small, very specialist, flight test team.
G